Surfaces of materials of silicon, hard disks, mother glasses for liquid-crystal displays, semiconductor devices, and so forth need to be flat, and accordingly are polished by a loose abrasive process using a polishing pad. The loose abrasive process is a method for polishing a work surface of a workpiece, while supplying a slurry (polishing liquid) containing abrasive grains between the polishing pad and the non-polished object.
As for a polishing pad for semiconductor devices, the surface of the polishing pad is required to have: opened pores for retaining abrasive grains; a hardness for keeping the flatness of the surfaces of the semiconductor devices; and an elasticity for preventing a scratch on the surfaces of the semiconductor devices. As polishing pads that meet these requirements, there have been utilized polishing pads which have a polishing layer produced from a urethane resin foam.
The urethane resin foam is generally molded by curing a prepolymer containing a polyurethane bond-containing isocyanate compound through a reaction with a curing agent (dry method). Then, the foam is sliced into a sheet shape, and thus a polishing pad is formed. In the case of a polishing pad having a hard polishing layer molded by the dry method as described above (hereinafter, sometimes abbreviated as a hard (dry) polishing pad), relatively small cells of a substantially spherical shape are formed inside the foam during the curing and molding of the urethane resin. Hence, on the polishing surface of the polishing pad formed by the slicing, opened pores (openings) are formed, which can retain a slurry during polishing.
Conventional mainstream urethane resin foams serving as a material of polishing pads for semiconductor devices have a cell diameter of 100 μm or less, and mostly have around 30 μm (Patent Literature 1). Moreover, the mainstream urethane resin foams have an A hardness of 70 degrees or more, a D hardness of 45 degrees or more (Patent Literatures 2 and 3), and a density is 0.5 g/cm3 or more (Patent Literature 1). As to the elasticity, the storage modulus is mainly several hundreds of MPa or more (Patent Literature 4). The modulus of longitudinal elasticity (Young's modulus) is mainly 500 MPa or more (Patent Literature 5).
Moreover, other than the above mainstream urethane resin foams, urethane resin foams have been improved in physical properties from the viewpoints of bulk density, A hardness, and hard segment content (HSC) (%) in order to optimize the degree of wearing and to stabilize the polishing performance (Patent Literature 6). Further, there are also reports on polishing pads, which are adjusted to have a storage modulus within a predetermined range in order to reduce occurrence of a scratch (Patent Literatures 7, 8).
In addition, there is a report on a polishing pad, which is improved in the A hardness and the modulus of compression elasticity by having a phase-separated structure made clear by: obtaining component amounts of a crystalline phase (L), an interface phase (M), and an amorphous phase (S) and the spin-spin relaxation times (T2) thereof from free induction decay signals (FID) in pulsed NMR; and then optimizing the T2 and the component amount of M (Patent Literature 9). Further, there is a report that, by adjusting a ratio of a hard segment present in a foamed polyurethane in pulsed NMR measurement to 55 to 70%, a polishing pad is made to exhibit properties such as being hard as well as being easily broken under tension with little elongation and has an enhanced dressing property while maintaining the high hardness (Patent Literature 10).